1. Field of the Invention
The present invention relates to a digitization apparatus used for digitizing a signal level of an analog signal, time intervals of pulse signals, etc.
2. Description of Related Art
One example of such a digitization apparatus is a TAD-type A/D converter (referred to simply as “TAD” hereinafter) all the parts of which are constituted by a digital circuit.
As described, for example, in Japanese Patent Application Laid-open No. 5-259907, the TAD includes a pulse delay circuit constituted by a plurality of delay units connected in series or in ring form. The delay time of these delay units depends on an operating voltage thereof. The TAD outputs the number of the delay units which a pulse signal passes through in a predetermined measuring time period as digitization data (A/D converted data) representing a voltage of an analog input signal applied to these delay units as their operating voltage.
That is to say, the TAD is configured such that a traveling speed of the pulse signal traveling through the pulse delay circuit is varied depending on the analog input signal applied to the delay units as their operating voltage, and this traveling speed is measured by counting the number of the delay units which the pulse signal passes through in the predetermined measuring time period.
The TAD can be used also as a time measuring apparatus capable of outputting data representing a length of elapsed time in digitized form, if a constant voltage signal is applied to the delay units as the analog input signal to set the delay time of the delay units at a known constant value.
In the TAD, to increase the resolution of its output digitization data (A/D converted data), the delay time of each one of the delay units has to be made shorter, or the measuring time period has to be made longer when the TAD is used as an A/D converter apparatus.
However, making the delay time of each delay unit has limitations, because the delay time is determined by a miniaturization level (CMOS design rule) of elements constituting the delay unit (gate circuits such as an inverter, for example). In addition, when it is required to drive the TAD by a low voltage for electric power saving, the delay time of the delay unit is restrained by this low driving voltage.
Furthermore, in a system using the TAD for performing A/D conversion at high speed (several MHz to several tenth of MHz, for example), it is not possible to lengthen the measuring time period to increase the resolution.
Incidentally, there is known a parallel-type (or flash-type) A/D converter capable of performing A/D conversion at high speed. However, the parallel-type A/D converter has to produce a number of reference voltages by use of analog circuits depending on the resolution required. Accordingly, in the case of the parallel-type A/D converter, increasing its output resolution causes its circuit scale to increase, and accordingly causes its production cost to increase.